Nucleotide-binding proteins play an extremely important role as regulators of genomic and proteomic function. Examples of nucleotide binding proteins include G proteins, which act as coupling factors in association with certain receptors; protein kinases, which transfer a phosphate group to target proteins; non-protein kinases, such as hexokinase, which are involved in the metabolic pathways within cells; proteins utilizing the energy stored within nucleotide-based molecules such as ATP; etc.
Protein kinases are the enzymes responsible for catalyzing the transfer of a γ-phosphoryl group from ATP to the hydroxyl group of serine, threonine or tyrosine residues in peptides, polypeptides, and proteins in a process known as “phosphorylation.” Protein phosphorylation is a ubiquitous regulatory mechanism in eukaryotic cells, where it is of central importance in controlling cell function, growth and differentiation. A protein kinase that phosphorylates, for example, tyrosine residues in its substrates is termed a protein-tyrosine:ATP phosphotransferase, or, more commonly, a tyrosine (or Tyr) kinase. The eukaryotic protein kinases make up a large superfamily of related proteins. They are related by virtue of their kinase domains (also known as catalytic domains), which consist of approximately 250-300 amino acid residues. The kinase domains that define this group of enzymes contain 12 conserved subdomains that fold into a common catalytic core structure. See, e.g., Hanks and Hunter, FASEB J. (1995) 9(8):576-96.
Eukaryotic protein kinases can be classified on the basis of their sequence, substrate specificity and regulation. One major subdivision is between Ser/Thr kinases and the Tyr kinases. Yeast have numerous Ser/Thr kinases, many of which have readily recognizable counterparts in all higher organisms, but very few dedicated Tyr kinases (an example of a yeast Tyr kinase is Swe1 from Saccharomyces cerevisiae and its homolog in S. pombe Wee1). By contrast, many signaling pathways of multicellular organisms depend on two large and important Tyr kinase families, the receptor-Tyr kinases which have intracellular Tyr kinase domains, and the Src family of cytoplasmic Tyr kinases. There are also dual-specificity enzymes, present in both unicellular and multicellular eukaryotes, such as the mitogen-activated protein kinase kinases (MAPKKs).
Overexpression and/or mutation of certain kinases in tumor cell is believed to upregulate a number of cell cycle and anti-apoptosis pathways leading to subversion of cell cycle checkpoints and enhanced cancer cell survival and metastatic potential. Conversely, inhibition of these kinases may reverse the aberrant signaling in receptor-overexpressing cells and may result in growth arrest and/or tumor cell death. Thus, it is no surprise that kinases have been considered important targets for the identification of therapeutics. See, e.g., Bishop et al., Trends Cell Biol (2001) 11(4):167-72.